Defeating the Air Conditioning Bogeyman
A/C Pressure and Temperature
Automotive repair professionals have a lot of “Bogeymen” we have to deal with. There’s the Bogeyman who makes electrical circuits function. There’s the Bogeyman involved with CAN communication. We have a Bogeyman for A/C too. In nearly every system that works but you’re just not sure why it works, there’s a Bogeyman involved. I like to try to shoo away the Bogeyman, or maybe shine a light on him so readers can see he’s not so scary after all. Then we can get back to work with some understanding and some peace from the monsters.
Today, we are shining the flashlight at the A/C Bogeyman. Remember learning about A/C in tech school? Remember hearing big words like Thermodynamics? School is back in session.
Air Conditioning Components
We’re gonna start with some light lifting. All A/C systems have much in common. They all have very similar parts and they all have two sides where pressure and temperature work their magic together. Check out this diagram showing some basic components and also the state of the refrigerant pressure/temperature between the “high side” and “low side.”
Let’s talk components. They should change the phrase “there are a lot of ways to skin a cat” to “there are a lot of ways to compress a gaseous refrigerant, condense it to a high-pressure liquid, and throttle it to a low-pressure liquid to boil and remove heat from a heat sink.” We’re not going to get granular and talk about how each type of compressor works, just that they compress. We are also staying away from control devices, like sensors and switches.
Compressor: This is usually a belt–driven pump; its job is to pull refrigerant in and compress it into a high-pressure vapor. It may have an electromagnetic clutch to engage or disengage it, or it may have a spill valve to control flow. On hybrid/EV vehicles this will likely be driven by an electric motor instead of a belt.
Condenser: This is a big finned heat exchanger, usually mounted right in front of the radiator. This is where the high-pressure vapor goes right after the compressor. Air passes through the fins to cool and condense the high-pressure vapor and turn it into a high-pressure liquid. Air is either motivated by cooling fans, or by ram air as the vehicle is driving.
Receiver/Dryer: This is a reservoir for the high-pressure liquid. It stores and filters the refrigerant and also has a desiccant to remove water from the refrigerant. This unit is typically mounted in the condenser.
Thermal Expansion Valve or Fixed Orifice: This is where the refrigerant goes when it is compressed, filtered, and condensed. This is a small orifice that the high-pressure liquid is pushed through, turning it into a low-pressure liquid. Think of it like a can of spray paint. There is high-pressure liquid inside the can, the nozzle at the top is the TXV—when you press down, the high-pressure liquid sprays out as a low-pressure liquid. If you’ve ever spray painted your hand you know the paint coming out is colder than the can. Thermal Expansion Valves (also called TXV, or just Expansion Valves) respond to the need for refrigerant by varying the size of the opening based on the temperature of the evaporator where the valve mounts.
See here a diagram of a TXV. The sensing bulb is full of a gas that expands when warm and presses down on the diaphragm in the valve. This pushes the valve open and allows more high-pressure liquid to spray into the evaporator as a low-pressure liquid, increasing the cooling capacity. Then as the evaporator cools, the gas from the sensing bulb contracts and the valve is allowed to close. Newer TXVs have all this integrated into a single block, you can’t see the sensing bulb anymore, but they function the same.
A Fixed Orifice performs the same job as a TXV, but does not vary in size at all, the manufacturer simply decides what a good size would be for most situations and lets it ride. A Fixed Orifice can be anywhere in the line between the condenser and the evaporator (hunt for it in the service information).
Evaporator Core: This is the heat sink inside the vehicle. The low-pressure liquid is sprayed from the TXV or Fixed Orifice into the evaporator. Here it evaporates! It turns into a low-pressure vapor again. Since the refrigerant is not under pressure it boils into a gas. It’s that pressure decrease that puts the “brrr!” in the A/C system.
Lines and Hoses: These are the conduits that take refrigerant in its varying states to where it needs to be; these should offer no resistance to flow at all, they’re simply tubes that should flow freely.
How the Components of an A/C System Function
Now that we understand what each component’s job is, let’s talk a bit about why they’re doing what they’re doing. This whole system is designed to take advantage of one thing: the pressure/temperature relationship of refrigerant. We are going to use R-134a refrigerant as our example—it’s still the most commonly used refrigerant, and the new fangled R1234yf has a pretty similar pressure/temp profile.
The refrigerant cycle can be described as having two parts, the high side and the low side. The high–pressure side is the path of the refrigerant after the compressor. It includes the compressor, condenser, dryer, lines, and technically a part of the TXV. The high-pressure side is where all the hot stuff is. The low-pressure side is the other part of the TXV, the evaporator core, and the line going back into the compressor. The low-side is where all the cold stuff is.
Download and print the pressure/temp chart at the bottom of this article for a reference. You can see that at 68°F the refrigerant is essentially 68 PSI. This means on a 68°day you will see about 68 PSI of static pressure in the system—static pressure is when the refrigerant has equalized so both the high side and the low side are the same pressure.
Now if you were to squeeze that 68 PSI until it gets to about 270 PSI (paging Dr. Compressor!) then you’ll see that the temperature raises to about 152°F. The compressor is literally jamming more molecules of refrigerant into a smaller area; they ping around and create heat. I don’t care what you’re squeezing, everything gets warmer when it is compressed. Things that are a gas at ambient temp/atmospheric pressure have a more useful range of pressures and temperatures that we can exploit.
The Refrigerant Cycle
So we have some hot gas, time to pull some of that heat out via the condenser to turn it into a high-pressure liquid. Leaving the condenser you will have maybe 130° high-pressure liquid, putting us at about 200 PSI. This is our high side pressure. The 200 PSI liquid jams up against the TXV. Remember that spray paint can? Here it is throttled down to about 30 PSI—checking the chart it looks like 30 PSI translates into about 34°F. That’s some chilly air! From here the low-pressure gas is pulled back into the compressor via the suction (low-side) line to start the process all over.
A quick note about condensers: they should only drop about 15-35 degrees of temp across them. If you see 50 degrees drop it’s not because the condenser is doing a really good job removing heat—more likely the condenser is clogged. Either the clog is acting like another expansion valve or the refrigerant is spending way too long in the condenser.
Congratulations, you just mastered thermodynamics! You understand the components of the A/C system and the refrigerant cycle!
So, how do we use the pressure and temperatures to fix cars? Let’s keep going to find out.
A/C Pressure Diagnostics
Static Pressure: This is always the first step in diagnosing an A/C cooling complaint. Check the ambient air temp, refer to the pressure/temp chart, check the pressure. If the pressure is low, you have low refrigerant charge—evacuate and recharge the system. If the pressure is high you have a non-condensable gas like nitrogen or oxygen in there or a drastic overcharge—evacuate and recharge the system. Basically, if you are ever in question about how much refrigerant is in the system, or what is in the system, evacuate and recharge, start fresh. Refer to the service manual for refrigerant type and capacity.
Normal Operation: High side is usually at least double of what static pressure was, plus 100 PSI if it’s humid. So at 68 PSI your rule-of-thumb pressure range would be 140-240 PSI.
Low side is usually 30-40 PSI, unless it is very hot; then it might be higher until the evaporator can maintain a colder temp. Off idle on a TXV–style vehicle, you will see the pressure drop and get as low as 10 PSI. A Fixed Orifice will usually keep the low side steady between 30-40 PSI regardless of RPM.
High Side Too High, Low Side Normal: If the high side approaches 350-400 PSI then the condenser is unable to rid itself of enough heat to turn the high-pressure gas to a liquid. Make sure the fins are not obstructed. Make sure the cooling fans come on and blow towards the engine. Fans that blow backward will push heat from the radiator and the engine into the condenser, rather than cooling the condenser down.
Low Side Too Low, High Side Normal: If the low side is too low then the compressor is pulling on the suction side but there is not enough refrigerant flow to feed it. There is either an obstruction in the evaporator, the suction line, or the TXV has failed. Before replacing anything put the system into a deep vacuum for several hours to try to boil off the water in the system, ice can form in the TXV and develop this type of complaint when there is moisture in the system.
Low and High Side Low: If when the compressor engages the low side drops but the high side does not rise, or maybe the high side drops as well, then there is an obstruction between the compressor and the high side pressure port. All that refrigerant is backing up somewhere, but your gauge can’t see it.
Low side High, High side Low: If the high side and low side do not change from static pressure when the compressor is engaged, then the compressor is not pumping and has failed.
Pressures Normal, not cooling: Assuming the blend doors are working normally the system might be insulated from the inside. Something is keeping that cold low side refrigerant from making the evaporator fins cold. Stop-leak can do this, so can an excessive amount of refrigerant oil.
Diagnosing A/C Issues
Once you have an idea what’s going on with your pressures, it’s time to start some hands-on diag. This is done with temperature–sensing equipment, either a temp gun, a probe, or with a thermal camera. You can also generically use your hands, they will tell you if something is hot or not, just without the fancy digital readout. If you are using a temp gun, note that measuring extruded aluminum lines can give you false readings; cover the line in masking tape and measure the temp of the tape for better accuracy. I prefer to use a thermal probe, that will be the most accurate. A temp gun is useful if you remember to take it with a grain of salt and a bit of skepticism.
Air Conditioning Temperature Readings
We know what the pressure and temperature should be normally. If you suspect an obstruction anywhere in the system, it’s time to check line/component temp. Here are some temps you should expect to see on a normal system at about 68°F.
Discharge line from compressor: This is the hottest part of the system—it should be 15-30 degrees hotter than the liquid line coming from the condenser. At 68° expect anywhere from 130-175°F depending on humidity.
Along the condenser: You should have a gradual drop in temp from the discharge line to the liquid line, you should not drop more than 50 degrees though, or the condenser may be plugged. Expect 15-30–degree drop.
Liquid line to the expansion valve: This is the “warm” line—you will see about 15-30 degrees cooler here than the discharge line. At 68°F, expect that you will have 110-140°F. You should have an even temp all the way to the expansion valve.
Expansion valve: The liquid line going in will be your 110-140° temp, the suction line coming out will be 32-42° almost regardless of the ambient air temp, once the evaporator has a chance to maintain its’ chill.
Common A/C Problem Areas
Check the temperature drop across the condenser—50–degree drop is a problem.
Check the temp along all the lines—you should not have any drop at all in the middle of a line, unless that’s where the Fixed Orifice is of course.
If the temperature of the lines is lower than suggested by the pressure on your gauge, then look for stop-leak or excessive refrigerant oil in the system.
Make sure to download the temp/pressure chart and graph, hang them up on your box for quick reference. If you run into a weird one that you can’t seem to get your head around, then give us a call here at Identifix. Happy wrenching!